add the *.VC.db to unreal engine ignore

as of visual studio 2015 update 2 this file stores the intellisense database.
this file is typically huge and shouldn't be included in a repository.

UnrealEngine.gitignore

 # Visual Studio 2015 user specific files
 .vs/
 
+# Visual Studio 2015 database file
+*.VC.db
+
 # Compiled Object files
 *.slo
 *.lo